Departing U.S. Sen. John F. Kerry thanked supporters in a bittersweet speech Thursday, remarking on his accomplishments as senator and criticizing the polarized state of national politics.

Mr. Kerry, a Democrat, is leaving the Senate after 28 years to become secretary of state to President Barack Obama. Speaking to students and other supporters at College of the Holy Cross, he slammed the bitter partisanship in Congress and the role of special interests in politics.

“We cannot be a country... that has lost our willingness to work together for the greater good, for the common good, and not always have a perpetual 24-7 political campaign,” he said.

“And we cannot be a country that allows so much money to come from such special interests that it crowds out the voices of average Americans.”

The five-term senator said leaders in Washington, D.C., should work together to solve problems, setting an example for leaders around the globe.

“The most important tool I need as the nation’s top diplomat, to be able to go out and leverage the kinds of behavior that we want, is credibility,” he said.

Mr. Kerry’s tenure as chairman of the Senate Foreign Relations Committee often took him to other countries, but he said he stayed committed to Massachusetts throughout.

He recalled how he went to Burma in December 1999 to meet Aung San Suu Kyi, the pro-democracy political leader then under house arrest. Soon after he landed there, he learned of the warehouse fire that had just killed six Worcester firefighters.

“It didn’t take me more than 10 seconds to say ‘I’ve got to find a plane, I’ve got to get home,’ ” Mr. Kerry said.

He called serving in the Senate “the finest privilege anybody could ever hope for” and said he would miss the constituent work of facilitating development projects, meeting local families, marching in parades and helping people who turned to his office when they didn’t know where else to turn.

The crowd, hundreds strong, applauded several times during his speech. Mr. Kerry shook hands and posed for a few photos after his remarks but left without talking to reporters.

In Worcester, Mr. Kerry was closely involved in negotiations with freight rail company CSX Corp.; the talks eventually led to a deal that expanded CSX’s freight operations in Worcester while freeing up space for more commuter trains to Boston.

He also brought federal dollars to Central Massachusetts, including for the major renovation of Union Station.

Mr. Kerry’s elevation to the State Department robs Massachusetts of the seniority it enjoyed for many years when Mr. Kerry and the late Edward M. Kennedy served in the Senate together. The state is now represented by Democratic freshman Sen. Elizabeth Warren. After Mr. Kerry officially resigns today, interim senator William “Mo” Cowan will take his place.

“It’s really a big loss,” said William J. Eddy, a city councilor who has worked on Democratic campaigns in the past. “Washington’s a seniority-based system and (Mr. Kerry) spent 28 years down there for us. You combine that with losing Sen. Kennedy, it’s an awful lot of seniority and clout that we’ve lost in the last three or four years.”

Mr. Kerry, who also scheduled farewell events in Springfield and Boston on Thursday, told the Worcester crowd that his path to becoming a diplomat began at age 11, when he moved to Germany because his father was in the Foreign Service. As a boy, he learned the difference between democracy and totalitarianism.

U.S. Rep. James P. McGovern, D-Worcester, in introducing the senator at Holy Cross, set the bar high for him.

“I want him to go to the State Department and end the war in Afghanistan…. I want him to lead the effort to end hunger in the world and end extreme poverty,” Mr. McGovern said. “I believe that he can change the world.”

Mr. Kerry will travel extensively in his new post, but he vowed not to abandon his home state.

“I am not leaving Massachusetts,” he said. “I’m going to continue to fight for this great state I love.”

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